My 8 y.o. son has CAH and that has happened to him, too. First time was when he was about 4 y.o. and second time was when he was 7. Rashes covered his entire body, first in his chest and back, then it spread to his face and limbs. He complained a bit about the itch but generally didn’t mind it at all. It lasted for about 3 days, then gradually disappeared. I didn’t give him anything since he seemed fine aside from the rashes.  I just assumed that he’s probably allergic to something he ate, but after reading your post, it made me rethink if it’s somehow connected to CAH. Or it can also be a coincidence that both our son has CAH and allergic to something, I don’t know :)

PS/ I think it’s antihistamine :) My other daughter has a lot of food allergies and I give her that med whenever she’s got an allergy attack.